Torque sensor of tension difference type for pulley-belt driving system

ABSTRACT

A torque sensor for a pulley-belt driving system has a belt passed round a drive pulley and a driven pulley. The torque sensor has a frame having opposite ends respectively in contact with first and second parts of the belt, a beam secured to a stationary body and supporting the frame and strain detection means for detecting the strain of the beam.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a torque sensor used for detecting the torquein a pulley-belt driving system having a belt wound on drive and drivenpulleys.

2. Prior Art Statement

In the pulley-belt driving system, a belt is wound on a drive pulleydriven by a motor and on a drive pulley receiving motion from the drivepulley, and the motor motion is transmitted to the driven pulleyutilizing the tension in the belt. This system is used in various powertransmission systems, and recently is also used for driving robot handsor fingers.

The motion of the robot hand or finger requires accurate position andpower control. Therefore, for employing the pulley-belt driving systemfor the driving of a robot hand or finger, the torque of the system hasto be controlled accurately. To achieve this end purpose, accuratemeasurement of torque is necessary.

FIG. 5 shows an example of the prior art torque sensor for a pulley-beltdriving system.

Referring to FIG. 5, reference numeral 5 designates the pulley-beltdriving system, which comprises a belt 4 fixed to a drive pulley 2 at apoint and to a driven pulley 3 at a point respectively. The drive pulley2 is driven by a motor (not shown). When the motor rotates, the pulley 2moves to pull the belt 4 and its torque is transmitted to the drivenpulley 3. A finger 6 is mounted on the driven pulley 3, and with themovement of the driven pulley 3 the finger 6 performs a job on an object7.

The pulley-belt driving system 5 is provided with a pair of torquesensors 20. One of the torque sensors 20 provided to detect the tensionof the first (upper) part 4a of the belt 4 includes a beam 23 connectedto a stationary body 15 at one end, a tension pulley 21 provided on theother end of the beam 23, and a pair of strain gauges 25 provided on theopposite sides of the beam 23. The other torque sensor 20 provided todetect the tension of the second (lower) part 4b of the belt 4 includesa beam 24 connected to the stationary body 15 at one end, a tensionpulley 22 provided on the other end of the beam 24 and a pair of straingauges 26 provided on the opposite sides of the beam 24. The tensionpulley 21 and 22 are urged against the respective first part 4a andsecond part 4b of the belt 4.

The torque M of the driven pulley 3 is calculated by the equation (1):##EQU1## where, M: torque of the driven pulley,

R: radius of the driven pulley,

θ: inclination angle of the belt,

T₁ : tension in the first part 4a of the belt,

T₂ : tension in the second part 4b of the belt,

F₁ : force applied by the tension of the first part 4a of the belt tothe pulley 21,

F₂ : force applied by the tension of the second part 4b of the belt tothe pulley 22,

k: proportionality constant,

V₁ : output from the strain gauges 25,

V₂ : output from the strain gauges 26.

FIG. 6 shows another example of the prior art torque sensor. This torquesensor 30 includes a tension sensor 31 mounted on the first (upper) part4a of a belt 4 to detect the tension T₁ and a tension sensor 32 mountedon the second (lower) part 4b to detect the tension T₂. Referencenumeral 11 designates a tension pulley. In this case, the torque Mgenerated at the driven pulley 3 is given as

    M=R(T.sub.1 -T.sub.2)                                      (2)

The above prior art torque sensors, however, have drawbacks. Morespecifically, the torque sensor 20 shown in FIG. 5 requires four straingauges and two beams, so that its construction is rather complicated. Inthe torque sensor 30 shown in FIG. 6, the two tension sensors movetogether with the belt 4. Therefore, special consideration must be givento the wiring for obtaining the output signals from the tension sensors31, 32 to avoid the damage of the wires during the motion.

Further, in the examples of FIG. 5 or 6, the tension difference (T₂-T₁), or the force difference (F₂ -F₁) that is necessary for derivingthe torque M of the driven pulley 3 is not obtained directly, but it isobtained after detecting T₁, T₂, F₁ and F₂ independently for processingdetected signals. This means that additional operations are required.

OBJECT AND SUMMARY OF THE INVENTION

This invention has been completed with the above problems in mind, andits object is to provide a torque sensor of tension difference typesuitable for a pulley-belt driving system, which is simple inconstruction and can directly detect the driven pulley torque throughsimple signal processing.

To attain the above object of the invention, there is provided a torquesensor for a pulley-belt driving system having a belt passed round driveand driven pulleys, and which comprises a frame for urging the belt inopposite directions, a beam secured to a stationary body and supportingthe frame, and strain detection means for detecting the strain of thebeam.

The tension difference between the two sides of the belt is proportionalto the displacement of the frame, which in turn is proportional to theflexing of the beam. Thus, the transmitted torque difference between thetwo sides of the belt can be readily obtained from the value of the beamstrain detected by the detection means.

The above and other objects and features of the invention will becomemore apparent from the following detailed description with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an embodiment of the torque sensoraccording to the invention;

FIG. 2 is a graph showing the relation between the applied torque at ajoint of a robot finger and the output of a torque sensor according tothe invention;

FIG. 3 is a front view showing an example of application of theinvention, in which torque sensors according to the invention areincorporated in a finger of a multi-finger hand of a robot;

FIG. 4 is a plan view showing the robot hand shown in FIG. 3;

FIG. 5 is a schematic view showing a prior art torque sensor; and

FIG. 6 is a schematic view showing a different prior art torque sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows the basic construction of a torque sensor according to theinvention. Referring to FIG. 1, reference numeral 5 designates apulley-belt driving system. The system 5 comprises a belt 4 passed rounda drive pulley driven by a motor (not shown) and a driven pulley 3. Thebelt 4 is fixed at two points, one on the periphery of the drive pulley2 and the other on the periphery of the driven pulley 3. In this case,it is desirable that the belt 4 be wound once on each of the drive anddriven pulleys 2 and 3 and fixed at respective intermediate points ofthe wound belt 4 to the drive and driven pulleys 2 and 3 to increase therange of rotation of each of the drive and driven pulleys 2 and 3.

When the drive pulley 2 is driven by the motor, the torque of the drivepulley 2 is transmitted by the belt 4 to the driven pulley 3. A finger 6of a robot is mounted on the driven pulley 3, and with the movement ofthe driven pulley 3 the finger 6 performs a job on an object 7.

The pulley-belt driving system 5 is provided with a torque sensor 1. Thetorque sensor 1 includes tension pulleys 11 and 12. The tension pulleys11 and 12 are mounted on a frame 13. They may urge the respective first(upper) and second (lower) parts 4a and 4b either inwardly or outwardlyso long as they urge the belt parts 4a and 4b in opposite directions. Inthe illustrated case, the tension pulleys 11 and 12 urge the first andsecond parts 4a an 4b of the belt 4 toward each other, i.e., inwardly.The frame 13 is supported at an intermediate portion to one end of abeam 14 having the other end fixed to a stationary body 15 and,therefore, when the drive pulley 2 is rotated by the motor in aclockwise direction, for example, the lower (second) part 4b of the belt4 is stretched while the upper (first) part 4a of the belt is flexed. Asa result, the frame 13 is moved downwardly as shown by an arrow F₂ inFIG. 1. A pair of strain gauges 16 (i.e., 16a and 16b) are mounted onthe opposite side surfaces of the beam 14.

The tension pulleys 11 and 12 provided on the opposite ends of the frame13 serve to give tension to the belt 4 and reduce the friction with thebelt 4. In the case where the frame 13 has high rigidity in thedirection of moving the belt 4, it is less liable to deform in thebelt-moving direction during the movement of the belt 4. In this case,therefore, the belt 4 may be brought into sliding contact with the frame13 by omitting the tension pulleys 11 and 12.

When a torque M is generated in the driven pulley 3, a tensiondifference T₁ -T₂ arises between the tensions T₁ and T₂ of the first andsecond parts 4a and 4b of the belt 4.

Meanwhile, the forces F₁ and F₂ which are applied in a direction normalto the straight line connecting the centers O₁ and O₂ of rotation of thedrive and driven pulleys 2 and 3 are respectively proportional to thetensions T₁ and T₂ and are represented by:

    F.sub.1 =2T.sub.1 sin θ

and

    F.sub.2 =2T.sub.2 sin θ

where θ is the inclination angle of the belt 4.

Thus, denoting the radius of the driven pulley 3 by R, the torque M ofthe driven pulley 3 is expressed by: ##EQU2## where, k: proportionalityconstant,

V: output of stain gauges 16 (16a, 16b)

The force difference F₁ -F₂ is proportional to the displacement of theframe 13, and the displacement of the frame 13 is proportional to theflexing of the beam 14. Thus, the torque M of the driven pulley 3 isobtained in proportion to the outputs of the strain gauges 16a and 16b.Therefore, there is no need of detecting the tensions T₁ and T₂ orforces F₁ and F₂ independently.

FIG. 2 is a graph showing an experimentally obtained staticcharacteristic of the sensor according to the invention. Morespecifically, the graph shows the results of detection via a strainamplifier of the outputs of the strain gauges 16a and 16b of the torquesensor 1 representing the moment generated about the center O₂ ofrotation of the pulley 3 produced by locking the drive pulley 2 drivenfrom the motor and applying a weight to the finger 6. In the graph theabscissa represents the torque applied to the finger and the ordinaterepresents the torque sensor output. Substantially the same linearcharacteristic as the linear characteristic obtained from the aboverelation could be obtained.

FIGS. 3 and 4 illustrate an example of detection of the torque ofrespective links of a robot hand 40 with torque sensors 1 according tothe invention.

The robot hand 40 has a stationary link 41 and a finger 42. The finger42 has first to third links 43 to 45. The first link 43 is coupled by afirst joint 46 to the stationary link 41, the first link 43 and secondlink 44 are coupled to each other by a second joint 47, and the secondlink 44 and third link 45 are coupled to each other by a third join 48.The first joint 46 has a first joint frame 51 having a channel-shapedsectional profile, a shaft 52, free rotation pulleys 53 and 54 and afixed rotation pulley 55. The fixed rotation pulle 55 is secured to thefirst joint frame 51, and the free rotation pulleys 53 and 54 arerotatably supported on an end of the stationary link 41. The first link43 is secured to the first joint frame 51.

The second joint 47 has a second joint frame 57 having a channel-shapedsectional profile, a shaft 58, a fixed rotation pulley 62 and a freerotation pulley 61. The fixed rotation pulley 62 is secured to thesecond joint frame 57. The second link 44 is secured to the second jointframe 5,, and the shaft 58 is rotatably supported on an end of the firstlink 43.

The third joint 48 has a third joint frame 64 having a channel-shapedsectional profile, a shaft 65 and a fixed rotation pulley 66. The fixedrotation pulley 66 is secured to the third joint frame 64. The thirdlink 45 is secured to the third joint frame 64, and the shaft 65 isrotatably supported on an end of the second link 44.

On the drive side, first to third drive pulleys 71 to 73 are mounted ona stationary body 67 and are driven from first to third motors 74 to 76,respectively.

A belt 101 is passed round the first drive pulley 71 and the fixedrotation pulley 55 of the first joint 46, a belt 102 is passed round thesecond drive pulley 72 and the fixed rotation pulley 62 of the secondjoint 47, and a belt 103 is passed round the third drive pulley 73 andfixed rotation pulley 66 of the third joint 48. The belt 101 are fixedat a point to the pulley 71 and at a point to the pulley 55. Likewise,the belts 102 and 103 are fixed at respective two points to the pulleys72, 62 and to the pulleys 73, 66, respectively. The belts 101, 102 and103 are stretched by respective weights W₁, W₂ and W₃.

A beam 14 of a torque sensor 1 with strain gauge 16a and 16b provided onthe opposite side surfaces is secured at one end to each of thestationary, first and second links 41, 43 and 44. Each beam 14 supportsa frame 13 at its free end. The frame 1 has pulleys 11 and 12 at theopposite ends. The pulleys 11 and 12 urge the associated one of thebelts 101 to 103 from the opposite sides.

When the pulleys 71 to 73 are driven by driving first to third motors 74to 76, the torque of the pulleys 71 to 73 is transmitted by the belts101 to 103 to the first to third links 43 to 45 to cause rotationthereof about the respective shafts 52, 58 and 65. At this time, thetorque of the first to third links is readily obtained from the tensiondifference between the first and second parts of the respective belts101, 102, 103 as detected by the torque sensors 1 provided for therespective belts.

The torque sensor having the construction as described above accordingto the invention requires only two strain gauges i.e., only one half thenumber of strain gauges as the prior art torque sensor shown in FIG. 5,so that it is possible to simplify the construction and reduce the costof the torque sensor.

In addition, with the torque sensor according to the invention thestrain gauge is at a fixed position and does not move. Thus, unlike theprior art torque sensor shown in FIG. 6, the wirings connected to thetension sensors do not moved together with the belt, so that no specialmeans is required to allow for their movement.

Further, unlike the torque sensor shown in FIGS. 5 and 6, with thetorque sensor according to the invention the difference necessary fordetermining the torque is not calculated from independently detectedtensions T₁ and T₂ or forces F₁ and F₂ but is directly detected, so thatthe operational process is simplified.

What is claimed is:
 1. A torque sensor for a pulley-belt driving systemhaving a belt passed around a drive pulley and a driven pulley,comprising:a frame having opposite ends respectively in contact withfirst and second parts of said belt to urge said belt in oppositedirections, whereby said frame displaces in response to a tensiondifference between said first and second parts of said belt; means formeasuring the displacement of the frame, comprising a beam having oneend fixed to an intermediate portion of said frame at right anglesrelative to said frame and between said first and second parts of saidbelt, whereby said beam bends with a displacement of said frame; astationary body having the other end of said beam fixed thereto forsupporting said frame; and strain detection means provided on said beamfor detecting the strain of said beam.
 2. The torque sensor according toclaim 1, further comprising a pair of tension pulleys rotatablysupported by the respective opposite ends of said frame so as to be incontact with the respective first and second parts of said belt.